Reality Programming

By William H. Calvin

Published: September 28, 2003

CONSCIOUSNESS

A User's Guide.

By Adam Zeman.

Illustrated. 404 pp. New Haven:

Yale University Press. $29.95.

AN articulate, liberally educated neurologist at the University of Edinburgh, Adam Zeman has written columns for The Times of London and is an occasional commentator for the BBC and the co-author of a book on ethical problems in neurology. His new book covers many aspects of consciousness for general readers. His treatment of the disorders of knowledge is superb. If you were intrigued with ''The Man Who Mistook His Wife for a Hat,'' you'll appreciate the buildup to what Oliver Sacks described in that work. Zeman's much more subtle examples give you some appreciation for how seeing and describing can become disconnected from recognition and other forms of knowledge.

There have been a number of fine books on consciousness in the last dozen years, starting with Daniel Dennett's ''Consciousness Explained,'' which was written from the standpoint of a philosopher well versed in cognitive sciences and evolution. I am also fond of ''The Feeling of What Happens,'' by Antonio Damasio. Like Zeman, Damasio is a neurologist steeped in both literature and philosophy. But Zeman's ''Consciousness'' is the broader book, the one that could be used in an undergraduate humanities or psychology course to fill in the neuroscience background for readers coming to it for the first time. Indeed, Zeman first introduces his subject and then spends a hundred pages on neurobiology and human evolution before returning to consciousness. Readers impatient for consciousness per se can skim these chapters without losing the thread, though they are relatively painless introductions to what consciousness is built atop of.

Consciousness implies both awake and aware. ''Sleep, like wakefulness, is organized from the brainstem,'' Zeman writes. ''It has a hidden structure of its own: in the course of the night we cycle repeatedly from light sleep to deep, from deep to dreaming sleep. The brainstem continues to generate these rhythms after the destruction of the hemispheres, as, for example, in the 'vegetative state.' By contrast, death of the brainstem is almost always followed, within hours or days, by death, pure and simple.'' The nerves controlling the entire body pass through the brainstem, and some brainstem strokes injure these connections while leaving the patient surprisingly alert and aware. ''In these circumstances awareness may survive while almost all means of expressing it are lost, an unhappy state of affairs known as the 'locked-in syndrome.' Sufferers from this disorder usually retain the ability to make voluntary up and down movements of their eyes, and can use these to communicate.'' He adds that we can't deny ''the disturbing possibility'' that this disorder may affect more people than we can now properly recognize as being afflicted by it.

A couple of chapters after I read this, my 91-year-old mother suffered a similar brainstem stroke. There were periods when she could communicate only by moving her eyes and eyelids. Because she was sometimes able to get a few words out, it was obvious she was tracking what we said and was thinking ahead as usual; her consciousness was trapped in a body that would no longer obey her. We knew her wishes about such situations quite well, her sister having lingered five years in a similar state, but my mother was still competent to make her own decision if we could frame it for her limited ability to communicate. So her doctor told her that her situation was unlikely to improve, that we proposed doing nothing except comfort drugs and that she probably would die within a few days. Was that what she preferred? We thought she would have to communicate by blinking her eyelids. But she burst forth with her longest utterance since her stroke: ''You're the best doctor I ever had.'' Those turned out to be her last words. She died two days later.

Consciousness is, however, more than just the minimum requirements of awake and focused. Zeman explains that ''sensation becomes conscious only when it undergoes some further process -- when it encounters past associations, or is used to govern future action, or becomes the object of reflection or is felt to impact upon the self.'' There is an ''important link between consciousness and volition. . . . Willed or voluntary acts are those with aims of which we are conscious and are -- usually -- prepared to acknowledge.'' Consciousness, he concludes, ''bridges perception and action, the events we perceive and the ones we bring about.''

But consciousness is fragile, and ''however magical it may be, it is a physical affair: mundane requirements for oxygen and glucose, electrical equilibrium, clean blood and adequate sleep must be met in the brain -- or consciousness fails. . . . However coherent our experience and behavior may appear, they are prone to fragment under stress. . . . Faints, fits and intoxication all reveal that perception, memory, movement and speech are separable capacities.'' He adds: ''It may be arrogant to deny that consciousness can ever slip its moorings in the brain -- after all, much of the world's population believes firmly that it can -- but the evidence in favor of this happening is tenuous at best.''